The present invention relates to manufacturing honeycomb structures, and more particularly honeycomb structures made of composite material. The invention also relates to tooling suitable for this purpose.
Honeycomb structures, in particular those made of composite material, are advantageous in that they generally present good mechanical properties, while being light in weight. They can be used, for example, as furnace bottoms.
Several methods exist for manufacturing honeycomb structures out of composite material.
One such method is shown in FIGS. 1 to 6 and comprises a three-dimensional reinforcing fabric of carbon fibers 1 which is prepared by stacking carbon fiber plies 10 and binding them together by needling, stitching, or any other analogous technique (FIG. 1).
Cuts 11 in the form of parallel slits placed in a staggered configuration are made through the fiber fabric 1 perpendicularly to the XY planes of the plies 10, e.g. by means of a knife or by using a water jet (FIG. 2).
The fabric is then stretched in a Y direction perpendicular to the XZ planes of the cuts, as shown by arrows F in FIG. 3 so as to form cells 12. This provides a fiber preform constituting a honeycomb 2 (FIG. 4).
The preform 2 is held in the stretched position by means of tooling constituted by a graphite bottom plate 3 and graphite studs 4 received in the cells along opposite edges of the preform (that are opposite in the Y direction, see FIG. 5). The assembly constituted by the expanded preform 2 and the tooling 3, 4 is then placed in a furnace where the preform 2 is densified with carbon by chemical vapor infiltration (CVI). In well-known manner, densification serves to fill the pores in the preform. At the end of the densification stage, a honeycomb structure 5 of composite material is obtained (FIG. 6).
Such a method is described in detail in French patent application No. FR-A-2 691 923.
In practice, it has been found that the inside faces of the cells obtained in this way (cf. reference 6 in FIG. 6) are not perfectly smooth, and often present surface defects such as roughness which can lead to injury while the structure is being handled. To resolve this problem it is possible to machine said inside faces so as to eliminate the defects. Nevertheless such an operation is very awkward.
Another drawback of the above-mentioned method lies in the fact that the cells in the final product are not always entirely regular in shape, i.e. in particular the cells are not all of the same shape, which is not only harmful to overall appearance, but can also reduce the strength of the structure.
The present invention seeks to remedy the above-mentioned drawbacks.
To this end, the invention provides a method of manufacturing a honeycomb structure, the method comprising the steps of:
forming staggered cells through the entire thickness of a fiber fabric;
causing pegs to penetrate into respective ones of the cells, each peg having a cross-section of size smaller than that of the corresponding cell and being made of a material that is suitable for expanding;
expanding the pegs so that they fill the cells and exert pressure on the inside faces of the cells; and
shrinking the pegs and then withdrawing them.
When the pegs are said to be xe2x80x9ccaused to penetratexe2x80x9d into the cells and when they are said to be xe2x80x9cwithdrawnxe2x80x9d from the cells, these terms refer to relative movement between the pegs and the cells. Such operations can be implemented by moving the fiber fabric relative to the pegs which are held stationary, or on the contrary by moving the pegs while the fiber fabric remains stationary.
The pressure exerted by the expanding pegs against the inside faces of the cells eliminates the roughness initially present on said inside faces. It also enables the cells to take the shape of the pegs, thereby conferring a regular shape thereto.
Typically, the pegs are mounted on a plate, preferably loosely, thereby enabling them to move relative thereto, and they extend substantially perpendicularly to the plate.
The pegs can be expanded by heating them, e.g. by heating the assembly constituted by the plate, the pegs, and the fiber fabric in an oven.
The pegs can be made of a material comprising silicone or metal. If they are made of metal, the pegs are preferably coated with an anti-adhesive layer so as to make it easier for them to slide in the cells of the fiber fabric.
In a variant, the pegs can consist of inflatable bladders, with the pegs being expanded by injecting gas under pressure into each of said bladders, and with the pegs being shrunk by deflating said bladders.
In yet another variant, the pegs can consist in inflatable bladders each containing a gas, and the pegs are expanded in the same manner as for pegs made of silicone or of metal, i.e. by thermal expansion. More particularly, by heating the assembly constituted by the plate, the pegs, and the fabric, the gas(es) contained in the bladders expand(s) to inflate the bladders.
Preferably, the method of the invention further comprises, prior to the step of expanding the pegs, a step which consists in applying a backing plate having through holes in positions that correspond with the cells and the pegs, the backing plate being placed against the fiber fabric so as to hold said fiber fabric against the plate.
Advantageously, the method of the invention also comprises a step consisting in impregnating the fiber fabric with resin, which step is preferably implemented prior to forming the cells in the fiber fabric. The resin serves to consolidate the fiber fabric so as to hold the cells open, or in other words so as to hold the cellular fiber fabric in a stretched condition.
The resin can then be cured. Advantageously, when the pegs are expanded by heating, a single heating operation is performed both for expanding the pegs and for curing the resin.
A step can also be provided, after the pegs have been shrunk and withdrawn, for the purpose of densifying the fiber fabric.
The present invention also provides tooling for implementing the above-defined method, the tooling comprising a plate and pegs mounted in a staggered configuration on the plate, substantially perpendicularly thereto, and made of a material that is suitable for expanding and preferably possessing a high coefficient of expansion.